Projection lens with high resolution and reduced overall length

ABSTRACT

An exemplary projection lens includes, in this order from the magnification side to the minification side thereof, a negative lens group of negative fraction power, and a positive lens group of positive fraction power. The projection lens satisfies the formulas of: −2.5&lt;F 1 /F&lt;−1.5; and 1.3&lt;F 2 /F&lt;1.5, where F 1 , F 2 , and F respectively represent the effective focal lengths of the negative lens group, the positive lens group, and the projection lens.

BACKGROUND

1. Technical Field

The invention relates to projection lens and, particularly, relates to aprojection lens having a high resolution and a small overall length.

2. Description of Related Art

In order to obtain a sharp projection image and reduce a size ofprojectors, such as digital light processing (DLP) projectors, liquidcrystal display (LCD) projectors, or liquid crystal on silicon (LCOS)projectors, projection lenses with high resolution but short overalllength (the distance between the magnification-side surface of such aprojection lens and a surface of a spatial light modulator (SLM), e.g.,digital micro-mirror device (DMD), LCD panel, or LCOS panel, equipped ina projector facing the projection lens) are needed. Factors affectingboth the resolution and the overall length of the projection lens, suchas the number and position of lenses employed, the refraction powerdistributions of the employed lenses, and the shape of each of theemployed lenses, complicate any attempt at increasing resolution andshortening overall length of projection lenses. For example, reducingthe number of lenses can shorten the overall length of the projectionlens, but resolution will suffer, conversely, increasing the number oflenses can increase resolution, but increases overall length of theprojection lens.

Therefore, it is desirable to provide a projection lens which canovercome the abovementioned problems.

SUMMARY

In a present embodiment, a projection lens includes, in this order fromthe magnification side to the minification side thereof, a negative lensgroup having negative fraction of power, and a positive lens grouphaving positive fraction of power. The projection lens satisfies theformulas of: −2.5<F1/F<−1.5; and 1.3<F2/F<1.5, where F1, F2, and Frespectively represent the effective focal lengths of the negative lensgroup, the positive lens group, and the projection lens.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present projection lens should be better understoodwith reference to the following drawings. The components in the drawingsare not necessarily drawn to scale, the emphasis instead being placedupon clearly illustrating the principles of the present projection lens.Moreover, in the drawings, like reference numerals designatecorresponding parts throughout the several views.

FIG. 1 is a schematic view of a projection lens in accordance with anembodiment.

FIG. 2-4 are graphs respectively showing spherical aberration, fieldcurvature, and distortion occurring in the projection lens in accordancewith a first exemplary embodiment.

FIG. 5-7 are graphs respectively showing spherical aberration, fieldcurvature, and distortion occurring in the projection lens in accordancewith a second exemplary embodiment.

FIG. 8-10 are graphs respectively showing spherical aberration, fieldcurvature, and distortion occurring in the projection lens in accordancewith a third exemplary embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present projection lens will now be described indetail with reference to the drawings.

Referring to FIG. 1, the projection lens 100, according to an exemplaryembodiment, is opportunely used in LCOS projectors. Such an LCOSprojector commonly equips with an LCOS panel (not shown) that has asurface 99 facing the projection lens 100. The projection lens 100includes, in this order from the magnification side to the minificationside thereof, a negative lens group 10, and a positive lens group 20.The negative lens group 10 has a negative refraction of power, while thepositive lens group 20 has a positive refraction of power. Theprojection lens 100 satisfies the formulas of: (1)−2.5<F1/F<−1.5; and(2) 1.3<F2/F<1.5. Where F1, F2, and F respectively represent theeffective focal lengths of the negative lens group 10, the positive lensgroup 20, and the projection lens 100.

The formulas (1), (2) are adapted for distributing the fraction power ofthe lens groups 10, 20, to limit the overall length of the projectionlens 100, and control/correct aberrations occurring in the projectionlens 100 within an acceptable level. As for formula (1), F1/F>−2.5 isconfigured to limit the overall length of the projection lens 100, pluswiden the field angle of the projection lens 100; F1/F<−1.5 is adaptedto control aberrations, especially distortion, caused by the negativelens group 10. Formula (2) is adapted to efficiently correct aberrationscaused by the negative lens group 10, and to obtain a telecentricprojection lens 100 with short overall length. Specifically, if F2/F>1.3is not satisfied, aberrations may be insufficiently corrected, and theprojection lens 100 may lose the telecentric characteristic, conversely,if F2/F<1.5 is not satisfied, aberrations may be over corrected, andattempt at shortening the overall length of the projection lens 100 mayfail.

Also, the projection lens 100 satisfies the formula: (3) Lb/F>1.6, whereLb is the rear focal length of the projection lens 100 (the distancebetween the minifcation-side surface of the projection lens 100 and thesurface 99). This formula is for increasing the rear focal length Lb toprovide sufficient space to accommodate an arrangement of a polarizer98, a half-wave plate 97, and a polarization beam splitter (PBS) prism96 of the LCOS projector.

Opportunely, the projection lens 100 also satisfies the formula: (4)0.24<D12/F<0.5, where D12 is the distance between the lens groups 10, 20on the optical axis of the projection lens 100. In detail, D12/F<0.5 isfor controlling the overall length of the projection lens 100, andD12/F>0.24 is to provide sufficient space to allow focus adjustment ofthe projection lens 100.

Specifically, the negative lens group 10 includes, in this order fromthe magnification side to the minification side of the projection lens100, a first lens 11 of negative refraction power, a second lens 12 ofpositive refraction power, a third lens 13 of negative refraction power,and a fourth lens 14 of positive refraction power. In order toefficiently control aberrations caused by the negative lens group 10, atleast one of the four lenses 11˜14 has at least one aspherical surface(i.e., aspherical lens), whereas, in order to reduce cost of theprojection lens 100, spherical lenses are preferably used in theprojection lens 100. Therefore, the negative lens group 10 of thisembodiment includes only one aspherical lens, e.g., aspherical firstlens 11 or second lens 12, to balance the controlling of aberrations andcontribute to the reduction of cost.

The positive lens group 20 includes, in this order from themagnification side to the minification side of the projection lens 100,a fifth lens 21 of positive refraction power, a sixth lens 22 ofnegative refraction power, a seventh lens 23 of positive refractionpower, and a eighth lens 24 of positive refraction power. For similarreasons as above, the positive lens group 20 includes only oneaspherical lens, e.g., aspherical seventh lens 23.

More specifically, the projection lens 100 further includes an aperturestop 95. The aperture stop 95 is interposed between the fifth lens 21and the sixth lens 22 so as to block off-axis light rays from the sixthlens 22 entering the fifth lens 21, and thereby prevents too muchdistortion occurring in the projection lens 100 (the off-axis light raysare the main cause of distortion).

In order to control chromatic aberration occurring in the projectionlens 100, at least one of the lenses 11˜14 of the negative lens group 10satisfies the formula: (5) V1>55, and n1>1.5, where V1, n1 respectivelyrepresent the Abbe number and the refractive index of the at least oneof the lenses 11˜14. In addition, at least one of the lenses 21˜24 ofthe positive lens group 20 satisfies the formula: (6) V2>60, where V2 isthe Abbe number of the at least one of the lenses 21˜24.

Opportunely and specifically, except the aspherical lenses, e.g.,aspherical lens 11, 12, or 23, the lenses of the projection lens 100 isadvantageously a spherical glass lens to reduce cost of the projectionlens 100 and control chromatic aberration occurring in the projectionlens 100. As for the aspherical lenses, each of their surfaces is shapedaccording to the formula:

${x - \frac{{ch}^{2}}{1 + \sqrt{1 - {\left( {k + 1} \right)c^{2}h^{2}}}} + {\sum{Aih}^{i}}},$

where h is a height from the optical axis of the projection lens 100 tothe aspherical surface, c is a vertex curvature, k is a conic constant,and Ai are i-th order correction coefficients of the aspheric surfaces.

Detailed examples of the projection lens 100 are given below in companywith FIGS. 2-10, but it should be noted that the projection lens is notlimited to these examples. Listed below are the symbols used in thesedetailed examples:

F_(No): F number;2ω: field angle;R: radius of curvature;d: distance between surfaces on the optical axis of the projection lens100;Nd: refractive index of lens; andV: Abbe constant.When projecting an image, the image is modulated by the LCoS panel, andprojects from the surface 99, transmits through the polarizer 98, thehalf-wave plate 97, the PBS prism 96, the projection lens 100, andfinally projects onto a screen (not shown).

Example 1

Tables 1, 2 show the lens data of Example 1, wherein F=21.58 mm,F1=−38.85 mm, F2=30.425 mm, F_(No)=2.45; 2ω=57.4°, and the second lens12 and the seventh lens 23 is aspherical lens.

TABLE 1 d Surface R (mm) (mm) Nd V Magnification-side surface of the56.088 1.5 1.589   61.135 first lens 11 Minification-side surface 23.0412.111 — — of the first lens 11 Magnification-side surface of the 66.1794.045 1.6073 26.65 second lens 12 Minification-side surface of the−39.229 2.771 — — second lens 12 Magnification-side surface of the−79.672 1.5 1.571  50.8  third lens 13 Minification-side surface 13.1783.771 — — of the third lens 13 Magnification-side surface of the −35.773.156 1.744   44.786 fourth lens 14 Minification-side surface of the−24.669 5.28 — — fourth lens 14 Magnification-side surface of the 43.2543.481 1.589   61.135 fifth lens 21 Minification-side surface of −19.6180.15 — — the fifth lens 21 Surface of aperture stop 95 infinite 9.834 —— Magnification-side surface of the −25.202 1.5 1.8467 23.78 sixth lens22 Minification-side surface 32.223 3.46 — — of the sixth lens 22Magnification-side surface of the −49.8 7.1 1.5247 56.26 seventh lens 23Minification-side surface of the −16.129 0.246 — — seventh lens 23Magnification-side surface of the 72.513 7.946 1.6968 55.53 eighth lens24 Minification-side surface of the −29.765 2.15 — — eighth lens 24Magnification-side surface infinite 22 1.6477  33.848 of the PBS prism96 Minification-side surface infinite 2 — — of the PBS prism 96Magnification-side surface of the infinite 0.7 1.5184 61.7  half-waveplate 97 Minification-side surface of infinite 1 — — the half-wave plate97 Magnification-side surface of the infinite 0.7 1.523  58.57 polarizer98 Minification-side surface of the infinite 8 — — polarizer 98 Thesurface 99 infinite — — —

TABLE 2 Surface Aspherical coefficient Magnification-side k = −10.97392;A4 = 1.82963E−5; surface of the A6 = −9.26848E−8; A8 = 5.59189E−10;second lens 12 A10 = −1.98742E−12 Minification-side k = −22.43766; A4 =3.49E−6; A6 = 2.72E−8; surface of the A8 = 7.02E−11; A10 = −7.80E−13second lens 12 Magnification-side k = −79.16789; A4 = −1.08162E−4;surface of the A6 = 9.00E−07; A8 = −9.89E−9; seventh lens 23 A10 =3.47E−11 Minification-side k = 0.4065924; A4 = 1.07E−5; A6 = 8.96E−9;surface of the A8 = 2.79E−10; A10 = −2.67E−12 seventh lens 23

As illustrated in FIG. 2, the curves f, d, and c are respectivespherical aberration characteristic curves of f light (wavelength: 486.1nm), d light (587.6 nm), and c light (656.3 nm) occurring in theprojection lens 100 of Example 1. Obviously, spherical aberrationoccurring in projection lens 100 of Example 1 is in a range of: −0.1mm˜0.1 mm. In FIG. 3, the curves t and s are the tangential fieldcurvature curve and the sagittal field curvature curve respectively.Clearly, field curvature occurring in the projection lens 100 of Example1 is limited to a range of: −0.1 mm˜0.1 mm. In FIG. 4, distortionoccurring in the projection lens 100 of Example 1 is limited to bewithin the range of: −2%˜2%.

Example 2

Tables 3, 4 show the lens data of EXAMPLE 2, wherein F=20.57 mm,F1=−43.378 mm, F2=29.585 mm, F_(No)=2.44; 2ω=56.98°, and the second lens12 and the seventh lens 23 is aspherical lens.

TABLE 3 Surface R (mm) d (mm) Nd V Magnification-side surface of the56.37951 1.5 1.5163  64.142 first lens 11 Minification-side surface ofthe first 19.45174 2.784273 — — lens 11 Magnification-side surface ofthe 67.52031 4.063589 1.6073 26.65 second lens 12 Minification-sidesurface of the −35.39937  1.838138 — — second lens 12 Magnification-sidesurface of the −115.634   1.5 1.5225  59.8354 third lens 13Minification-side surface of the third 12.79318 4.061006 — — lens 13Magnification-side surface of the −33.31014  3.632139 1.6204  60.2896fourth lens 14 Minification-side surface of the −22.97251  5.274921 — —fourth lens 14 Magnification-side surface of the 47.65653 3.3159281.589   61.135 fifth lens 21 Minification-side surface of the fifth−19.51972  0.15 — — lens 21 Surface of aperture stop 95 infinite8.758488 — — Magnification-side surface of the −24.89886  1.5 1.846723.78 sixth lens 22 Minification-side surface of the sixth 30.708573.418172 — — lens 22 Magnification-side surface of the −50.35514  7.11.5247 56.26 seventh lens 23 Minification-side surface of the −15.63497 1.041066 — — seventh lens 23 Magnification-side surface of the 84.059077.926621 1.6968 55.53 eighth lens 24 Minification-side surface of the−28.60263  2.135659 — — eighth lens 24 Magnification-side surface of thePBS infinite 22 1.6477  33.848 prism 96 Minification-side surface of thePBS infinite 2 — — prism 96 Magnification-side surface of the infinite0.7 1.5184 61.7  half-wave plate 97 Minification-side surface of thehalf- infinite 1 — — wave plate 97 Magnification-side surface of theinfinite 0.7 1.523  58.57 polarizer 98 Minification-side surface of theinfinite 8 — — polarizer 98 The surface 99 infinite — — —

TABLE 4 Surface Aspherical coefficient Magnification-side k = −10.17326;A4 = 1.74E−5; A6 = −1.02E−7; surface of the A8 = 6.22E−10; A10 =−2.15E−12 second lens 12 Minification-side k = −19.16988; A4 = 1.81E−6;A6 = 3.9E−8; surface of the A8 = 3.7E−11; A10 = −6.46E−13 second lens 12Magnification-side k = −91.33926; A4 = −1.21802E−4; surface of the A6 =1.14E−6; A8 = −1.34E−8; A10 = 5.7E−11 seventh lens 23 Minification-sidek = 0.3093345; A4 = 8.74E−6; A6 = −4.76E−9; surface of the A8 =3.04E−10; A10 = −3.85E−12 seventh lens 23

As illustrated in FIG. 5, spherical aberration occurring in projectionlens 100 of Example 2 is limited to a range of: −0.1 mm˜0.1 mm. As shownin FIG. 6, field curvature occurring in the projection lens 100 ofExample 2 is limited to a range of: −0.1 mm˜0.1 mm. In FIG. 7,distortion occurring in the projection lens 100 of Example 2 is limitedto be within the range of: −2%˜2%.

Example 3

Tables 5, 6 show the lens data of EXAMPLE 3, wherein F=21.2 mm; F1=−47mm; F2=31.02 mm; F_(No)=2.46; 2ω=55.57°, and the first lens 11 and theseventh lens 23 are aspherical lenses.

TABLE 5 Surface R (mm) d (mm) Nd V Magnification-side surface of the58.051 2.41 1.5247 56.26  first lens 11 Minification-side surface of thefirst 11.667 2.202 — — lens 11 Magnification-side surface of the 21.4393.004 1.806  33.2694 second lens 12 Minification-side surface of the103.82  2.701 — — second lens 12 Magnification-side surface of the−21.595  1.501 1.6175 48.0786 third lens 13 Minification-side surface ofthe third 20.408 2.524 — — lens 13 Magnification-side surface of the30.179 3.302 1.6742 51.7521 fourth lens 14 Minification-side surface ofthe −51.502  6.5 — — fourth lens 14 Magnification-side surface of the41.495 2.383 1.6987 48.9334 fifth lens 21 Minification-side surface ofthe fifth −40.266  0.918 — — lens 21 Surface of aperture stop 95infinite 11.36 — — Magnification-side surface of the −19.698  1.5 1.846723.78  sixth lens 22 Minification-side surface of the sixth 73.743 1.214— — lens 22 Magnification-side surface of the 153.785  10 1.5247 56.26 seventh lens 23 Minification-side surface of the −20.171  0.2 — —seventh lens 23 Magnification-side surface of the 47.879 7.871 1.617860.4578 eighth lens 24 Minification-side surface of the −34.109  2 — —eighth lens 24 Magnification-side surface of the PBS infinite 22 1.647733.8482 prism 96 Minification-side surface of the PBS infinite 1 — —prism 96 Magnification-side surface of the infinite 0.7 1.5184 61.7  half-wave plate 97 Minification-side surface of the half-wave infinite 1— — plate 97 Magnification-side surface of the infinite 0.7 1.523 58.57  polarizer 98 Minification-side surface of the infinite 8.012 — —polarizer 98 The surface 99 infinite — — —

TABLE 6 Surface Aspherical coefficient Magnification-side k = 2.9896; A4= 1.42E−5; A6 = −8.47E−8; surface of the first A8 = 3.87E−10; A10 =7.12E−13 lens 11 Minification-side k = −0.8105889; A4 = 2.33E−5; A6 =−5.69E−9; surface of the first A8 = −1.99E−9; A10 = 2.06E−11 lens 11Magnification-side k = −748.1059; A4 = 2.68E−5; A6 = −1.04E−7; surfaceof the A8 = 1.76E−09; A10 = −4.39E−12 seventh lens 23 Minification-sidek = −0.4196788; A4 = 1.26E−5; surface of the A6 = 9.62E−8; A8 =−1.61E−10; seventh lens 23 A10 = 3.23E−12

As illustrated in FIG. 8, spherical aberration occurring in projectionlens 100 of Example 3 is limited to a range of: −0.1 mm˜0.1 mm. As shownin FIG. 9, field curvature occurring in the projection lens 100 ofExample 3 is limited to a range of: −0.1 mm˜0.1 mm. In FIG. 10,distortion occurring in the projection lens 100 of Example 3 is limitedto be within the range of: −2%˜2%.

In all, in Examples 1˜3, though the overall length of the projectionlens 100 is reduced, the resolution of the projection lens 200 ismaintained, even improved, since aberrations occurring in the projectionlens 100 are controlled to be in an acceptable range.

It will be understood that the above particular embodiments and methodsare shown and described by way of illustration only. The principles andthe features of the present invention may be employed in various andnumerous embodiment thereof without departing from the scope of theinvention as claimed. The above-described embodiments illustrate thescope of the invention but do not restrict the scope of the invention.

1. An projection lens comprising, in this order from the magnificationside to the minification side thereof, a negative lens group havingnegative fraction power, and a positive lens group having positivefraction power, wherein the projection lens satisfies the formulas of:−2.5<F1/F<−10.5; and 1.3<F2/F<1.5, where F1, F2, and F respectivelyrepresent the effective focal lengths of the negative lens group, thepositive lens group, and the projection lens.
 2. The projection lens asclaimed in claim 1, wherein the projection lens satisfies the formula:Lb/F>1.6, where Lb is the rear focal length of the projection lens. 3.The projection lens as claimed in claim 1, wherein the projection lenssatisfies the formula: 0.24<D12/F<0.5, where D12 is the distance betweenthe negative lens group and the positive lens group on the optical axisof the projection lens.
 4. The projection lens as claimed in claim 1,wherein the negative lens group comprises, in this order from themagnification side to the minification side of the projection lens, afirst lens of negative refraction power, a positive lens of positiverefraction power, a third lens of negative refraction power, and afourth lens of positive refraction power.
 5. The projection lens asclaimed in claim 1, wherein the negative lens group comprises at leastone aspherical lens.
 6. The projection lens as claimed in claim 1wherein the negative lens group comprises at least one lens satisfyingthe formula: V>55, and n>1.5, where V is the Abbe number of the at leastone lens, and n is the refractive index of the at least one lens.
 7. Theprojection lens as claimed in claim 1, wherein the positive lens groupcomprises, in this order from the magnification side to the minificationside of the projection lens, a first lens of positive refraction power,a second lens of negative refraction power, a third lens of positiverefraction power, and a fourth lens of positive refraction power.
 8. Theprojection lens as claimed in claim 7, further comprising an aperturestop, the aperture stop being interposed between the first lens and thesecond lens.
 9. The projection lens as claimed in claim 1, wherein thepositive lens group comprises at least one aspherical lens.
 10. Theprojection lens as claimed in claim 1, wherein the positive lens groupcomprises at least one lens satisfying the formula: V>60, where V is theAbbe number of the at least one lens.